1. Field of Invention
This invention relates to microwave filters and more particularly to an array of directional filters.
2. Description of the Prior Art
A man made satellite that orbits about the earth is often used to transmit a message to the earth. Typically, the message is transmitted by a transponder that is aboard the satellite.
In one type of transponder, the message is a modulated signal that has a frequency within one of twelve signal channels. The channels are bands of frequencies of approximately 36 MHz within a broad band that extends from 3.7 GHz to 4.2 GHz. There is usually a guard band of approximately four MHz between adjacent channels.
The transponder additionally includes twelve travelling wave tube amplifiers that respectively amplify message signals within the twelve channels. The outputs of the amplifiers are connected to an antenna through twelve band pass filters, respectively. The pass bands of the twelve filters are substantially equal to the twelve channels whereby the filters reject noise generated by the amplifiers. Therefore, each filter corresponds to a channel.
When an amplified message signal passes through a filter with a phase shift that is linearly proportional to the frequency of the amplified message signal, the filter provides an undistorted output. The rate of change of the phase shift with respect to the frequency is known as the group delay of the amplified message signal. When the phase shift is linearly proportional to the frequency, the group delay is constant.
When two filters, corresponding to adjacent channels, have their outputs connected together, there is usually an undesired interaction between the two filters. The interaction occurs because the adjacent channels are separated by only the four MHz guard band. The interaction causes a variation of the group delay of amplified message signals within the corresponding adjacent channels. Additionally, the interaction causes the connected filters to have distorted pass bands. To obviate the variation of the group delay and the distortion of the pass bands, the twelve filters are formed into first and second groups of six filters that are connected to first and second ports, respectively, of the antenna.
The first group of filters correspond to six alternate adjacent channels, whereby the second group also corresponds to six alternate adjacent channels. Accordingly, both of the antenna ports receive signals of alternate adjacent channels, thereby obviating the variation of the group delay and the distortion of pass bands. However, because the antenna has two ports, the design of the antenna is complex.